Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session GT: Biolocomotion V: Flapping and Flying I |
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Chair: Alexandra Techet, Massachusetts Institute of Technology Room: Long Beach Convention Center Grand Ballroom B |
Monday, November 22, 2010 8:00AM - 8:13AM |
GT.00001: Falling flexible sheets Silas Alben We use inviscid simulations to study falling flexible sheets in the two-parameter space of sheet density and bending rigidity. The basic behavior is a repeated series of accelerations to a critical speed at which the sheet flexes, and rapidly decelerates, shedding large vortices. The maximum and average speeds of the sheet are closely related to the critical flutter speed. The sheet trajectories also show persistent circling, quasi-periodic flapping, and more complex repeated patterns. [Preview Abstract] |
Monday, November 22, 2010 8:13AM - 8:26AM |
GT.00002: Fluttering dynamics of passive flexible wings Daniel Tam, John W.M. Bush We investigate the dynamics of passive flexible wings freely falling under the influence of gravity. Particular attention is given to elucidating the role of flexibility in gliding flight, specifically side-to-side fluttering motion. The effect of bending on the dynamics of fluttering wings is examined through an experimental investigation of deformable rectangular wings falling in water. We demonstrate that the elastic deformations induced by the flow strongly affect the flight characteristics, specifically the period and amplitude of the side to side fluttering motion as well as the descent rate. Our results suggest the existence of an optimal bending rigidity that maximizes the descent time for a particular wing geometry. Biological implications are discussed. [Preview Abstract] |
Monday, November 22, 2010 8:26AM - 8:39AM |
GT.00003: Falling with Style - Bat flight maneuvers Attila Bergou, Daniel Riskin, Gabriel Taubin, Sharon Swartz, Kenneth S. Breuer The remarkable maneuverability of flying animals results from precise movements of their highly specialized wings. Among these flyers, bats have evolved a particularly impressive capacity to control their flight. This adeptness is, in part, determined by bats' ability to modulate their wing shape through many independently controlled joints. However, the many-jointed wings of bats have higher inertia relative to their bodies compared with all other extant flyers. To understand the role that wing inertia plays in bat flight, we use a novel tracking algorithm to measure the kinematics of bats performing aerial flips. Using a dynamical model of a flying bat, we show how bats modulate their wings' inertia, usually a detriment to maneuvering, to supplement aerodynamic forces in performing flight maneuvers. [Preview Abstract] |
Monday, November 22, 2010 8:39AM - 8:52AM |
GT.00004: Effect of mass ratio for a flexible flapping wing during forward flight Haoxiang Luo, Fang-Bao Tian, Xi-Yun Lu During flight, insect wings typically deform under a combined aerodynamic force and wing inertia, whichever is dominant depends on the properly scaled mass ratio between the wing and air. To study the differences that the wing inertia makes in the aerodynamic performance of the deformable wing, a two- dimensional numerical study is applied to simulate the flow-- structure interaction of a flapping wing during forward flight. The wing section is modeled as an elastic plate that may experience nonlinear deformations while flapping. The effect of the wing inertia on lift, thrust, and power is studied for a range of wing rigidity and kinematic parameters such as the stroke plane angle and advance angle. It is found that the wing flexibility can dramatically increase the thrust without significantly losing lift or increasing the power input. Furthermore, the wings with low mass ratios could have much better efficiency than the wings with high mass ratios. The implication of the findings on insect flight will be discussed. [Preview Abstract] |
Monday, November 22, 2010 8:52AM - 9:05AM |
GT.00005: Thrust performance and wake structure of a pitching flexible plate at low aspect ratio Paulo Ferreira de Sousa, Hu Dai, Haoxiang Luo, James Doyle Thrust performance and wake structure are numerically investigated for a rectangular plate (AR = 0.54) that pitches around the leading edge in a free stream. The plate is flexible and it may undergo large displacement. The simulations employ a newly developed fluid-structure-interaction code based on a sharp-interface immersed boundary solver for the flow and a nonlinear finite-element solver for the elastic plate. Implemented on a Cartesian mesh, the flow solver allows us to capture the vortex dynamics of the wake accurately and efficiently. The mass ratio of the plate is low so that the deformation is solely caused by the hydrodynamic force. The results will be compared with the experimental result for the rigid plate from Buchholz and Smits (J Fluid Mech 603, 2008). Both the thrust level and power efficiency will be used to evaluate the performance of the plate, and the results will be compared with those for the corresponding rigid plate with the same effective pitching angle. The effect of the active pitching angle, the bending rigidity, and the Strouhal number will be presented. [Preview Abstract] |
Monday, November 22, 2010 9:05AM - 9:18AM |
GT.00006: Efficient flapping flight using flexible wings oscillating at resonance Alexander Alexeev, Hassan Masoud Using a fully-coupled computational approach that integrates the lattice Boltzmann and lattice spring models, we investigate the three-dimensional aerodynamics of flexible flapping wings at resonance. The wings are tilted from the horizontal and oscillate vertically driven by a force applied at the wing root. Our simulations reveal that resonance oscillations drastically enhance the aerodynamic efficiency of low-Reynolds-number plunging, and yield lift and lift-to-weight ratio comparable to the values typical for small insects. Within the resonance band, we identify two flapping regimes leading to the maximum lift and the maximum efficiency, which are characterized by different bending modes of flexible flapping wings. Our results indicate the feasibility of using flexible wings driven by a simple harmonic stroke for designing efficient microscale flying machines. [Preview Abstract] |
Monday, November 22, 2010 9:18AM - 9:31AM |
GT.00007: On passive wing response in hovering kinematics at low Reynolds number Albert Medina, Jeff D. Eldredge The aerodynamic role of passive wing mechanics in biological flight remains poorly understood. This computational study focuses on the effects of passive pitch response via chordwise flexibility in two-dimensional hovering kinematics. The wing consists of solid elliptical bodies with interconnecting torsional springs. Tests of a wing subjected to a nominally perpendicular freestream reveal new mechanisms for mean lift generation through symmetry-breaking. Additionally, chordwise flexibility in hovering kinematics accounts for greater average lift production and efficiency than an equivalent rigid wing. It is shown that an effective angle of attack can be defined that collapses the performance of the flexible wing to that of a rigid wing. Further, it is found that the performance of a flexible wing undergoing hovering maneuvers is less sensitive to pitch amplitude and phase. Finally, these results are put into context with recent studies of passive pitching of a rigid wing, and a more general class of passive wing behaviors is identified. [Preview Abstract] |
Monday, November 22, 2010 9:31AM - 9:44AM |
GT.00008: The effect of chordwise flexibility on flapping foil propulsion in quiescent fluid Sachin Shinde, Jaywant Arakeri Motivated to understand the role of wing flexibility of flying creatures during hovering, we experimentally study the effect of chordwise flexibility on the flow generated in quiescent fluid by a sinusoidally pitching rigid symmetrical foil with a flexible flap attached at the trailing edge. This foil produces a narrow, coherent jet containing reverse Karman vortex street, and a corresponding thrust. The thrust and flow is similar to that produced by a hovering bird or insect, however the mechanism seems to be different from known hovering mechanisms. Novelty of the present hovering mechanism is that the thrust generation is due to the coordinated pushing action of rigid foil and flexible flap. We identify the flow and vortex generation mechanism. This foil produces jet flows over a range of flapping frequencies and amplitudes. In contrast, the foil without flap i.e. with rigid trailing edge produces a weak, divergent jet that meanders randomly. Appending a flexible flap to the foil suppresses jet-meandering and strengthens the jet. Flexibility of flap is crucial in determining the flow structure. This study is useful in designing MAVs and thrusters. [Preview Abstract] |
Monday, November 22, 2010 9:44AM - 9:57AM |
GT.00009: Flexible Flapping Foils Catherine Marais, Ramiro Godoy-Diana, Jos\'e Eduardo Wesfreid Hydrodynamic tunnel experiments with flexible flapping foils of 4:1 span-to-chord aspect ratio are used in the present work to study the effect of foil compliance in the dynamical features of a propulsive wake. The average thrust force produced by the foil is estimated from 2D PIV measurements and the regime transitions in the wake are characterized according to a flapping frequency-amplitude phase diagram as in Godoy-Diana et al. (Phys. Rev. E 77, 016308, 2008). We show that the thrust production regime occurs on a broader region of the parameter space for flexible foils, with propulsive forces up to 3 times greater than for the rigid case. We examine in detail the vortex generation at the trailing edge of the foils, and propose a mechanism to explain how foil deformation leads to an optimization of propulsion. [Preview Abstract] |
Monday, November 22, 2010 9:57AM - 10:10AM |
GT.00010: Bendable ring flapping in a uniform flow Bo Young Kim, Soo Jai Shin, Hyung Jin Sung To understand flow-induced flapping motions of bendable objects, we numerically investigate dynamics of a pressurized elastic ring pinned at one point within a uniform flow by using an improved version of the immersed boundary method. The boundary of the ring consists of a flexible filament with bending stiffness, which can be modeled as a linear spring with spring constant k and initial unstretched length. The internal area of the ring is conserved through the penalty method. The flapping motion of the ring is decomposed into two parts: a pitching motion that includes flexible bending motion in the transverse direction, and a tapping motion in the longitudinal direction. For the Reynolds number of 100, resonance is observed at k $\sim $11, where k is normalized by the diameter of the undeformed ring, the speed of the upcoming flow and the fluid density. Across the resonance region, an abrupt jump in terms of the motion amplitudes as well as the hydrodynamic loads is recorded. In our simulation we observe bistable states, one stationary and another oscillatory, that coexist over a range of flow velocities. [Preview Abstract] |
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